Photochemical oxidation of dimethylsulphide to dimethylsulphoxide in estuarine and coastal waters

Abstract Dimethylsulphide (DMS) photo-oxidation and dimethylsulphoxide (DMSO) photoproduction were estimated in 26 laboratory irradiations of coastal samples from NE England (Tyne estuary) and W Scotland (Loch Linnhe and River Nant at Taynuilt). Pseudo-first order rate constants of DMS photo-oxidation (0.038 h −1 to 0.345 h −1 ) and DMSO photo-production (0.017 h −1 to 0.283 h −1 ) varied by one order of magnitude and were lowest in the coastal North Sea. Estuarine samples (salinity S  350 . Variations in a 350 explained 61% (R 2  = 0.61, n = 26) and 73% (R 2  = 0.73, n = 17) of the variability in DMS photo-oxidation and DMSO production, respectively. However, CDOM normalised photochemical rate constants increased strongly towards coastal waters exhibiting lowest CDOM absorbance, indicating water samples of marine character (S > 30) to be most reactive with respect to DMS photo-oxidation. Estimates of water column averaged DMS photo-oxidation rate constants, obtained by scaling to mean daily irradiance (July, NE England) and mid-UV underwater irradiance, were 0.012 d −1 , 0.019 d −1 , and 0.017 d −1 for upper estuary (S   30), at the lower end of previous observations. Comparing our water column averaged DMS photo-oxidation rate constants with estimated DMS losses via air-sea gas exchange and previously reported biological consumption implies that DMS photochemical removal is of only minor importance in our study area.